Arecoline, a cholinergic agonist, improves memory in patients with Alzheimer's disease when given by continuous intravenous infusion. Improvement follows an inverted U-shaped relation to dose, and is maximal at a dose at least ten-fold below that producing toxicity. Biopterin administration to some subjects with Alzheimer's disease elevated cerebrospinal fluid biopterin values to age-matched normal values. Biopterin appeared also to increase central dopamine turnover in these subjects, suggesting that biopterin deficiency may lead to impaired dopaminergic neurotransmission in a subset of Alzheimer's subjects. Administration of haloperidol, a dopamine antagonist, produced greater cognitive and motor effects in young than in aged healthy men, suggesting that responsivity of the brain dopamine system is reduced with age in humans. Brain dopamine turnover, as measured with debrisoquin method, is slightly reduced in aged compared to young healthy adult males.